Wireless communication increases rapidly and various ways to accommodate the increased number of users and amount of traffic are discussed and developed. FIG. 1 illustrates a typical deployment in a wireless communication network 1. A first network node 2, e.g. an evolved node B (eNB), provides wireless communication to a number of communication devices 5 (denoted user equipment, UE, in the following) within a coverage area 6, often denoted cell. Within the coverage area 6 of this first network node 2 further network nodes 3, 4 (e.g. remote radio units or access points) may be arranged having a respective coverage area 7, 8. This is a typical heterogeneous network deployment, wherein the further network nodes 3, 4 (e.g. pico nodes) are configured to transmit at a lower power than the first network node 2 (e.g. macro node). In such deployment, each network node 2, 3, 4 has its own cell identification (ID), a cell and a cell identification thus being mapped one to one.
In Long Term Evolution (LTE) standard a concept called shared cell (also denoted cell merge, combined cell or multi sector cell) is discussed. FIG. 2 illustrates such shared cell deployment. The shared cell 9 comprises (as in the deployment of FIG. 1) multiple network nodes 2, 3, 4, often denoted sectors in this context. The shared cell 9 may thus contain multiple sectors 2, 3, 4, and transmission to and reception from the UE 5 can be done by one sector or by multiple sectors. In the shared cell, the different sectors are allowed to use the same Physical Cell Identity (PCI) and thus all sectors are considered, by the UE 5, to be one single cell 9. With the shared cell concept the ID of one cell is, in contrast to the above deployment, mapped to several sectors. Using shared cell instead of the regular deployment (shown in FIG. 1) may give less capacity, since a reduced number of users can be scheduled simultaneously with fewer cell IDs. There are several benefits with the shared cell deployment, for example the reduction of coverage holes enabled by allowing multiple coverage areas within the same cell. Further, interference from data transmissions in downlink is reduced by using a subset of sectors and no handover is needed when the UE 5 moves from one sector to another using the same cell ID.
In a Third Generation Partnership Project (3GPP) LTE Release 8 system with the shared cell deployment, downlink channel quality measurements are made based on cell specific reference signals (CRSs). While data transmissions in the shared cell 9 may be transmitted in only a selected subset of all the sectors within the shared cell 9, CRS still needs to be transmitted in all sectors since other UEs than the intended receiver of the data transmission should also receive them. This may lead to a UE 5 seriously overestimating a channel if the CRS are received from sectors that are not within the set of sectors selected for data transmissions. This may in turn have a serious impact on downlink performance for this UE 5.
In particular, with currently existing solutions, the sectors selected for downlink data transmissions are typically chosen based on uplink channel measurements. When only a subset of the sectors are used for transmission there is a risk that the UE may still receive CRS transmissions from sectors not included in the subset of sectors chosen for data transmissions. As the channel estimates on the UE side are performed based on CRS receptions, this may lead to an overestimation of the downlink channel which will lead to link adaptation using a more aggressive choice of modulation and coding scheme which in turn will cause an increased probability of decoding errors in the UE.
Furthermore, the CRS channel estimations are also used for demodulation. In the demodulation erroneous channel estimates may have an even larger impact than when used for link adaptation only.